To prove that the owner of the car or at least the person registered to it really was driving the car at the time.

Now full disclosure, I don’t drive, but of my buddies that do, one received a ticket last year here in California in the mail that had a picture which clearly identified her (and me, I was in the passenger seat). If she fought the ticket, presumably they could pull this photo out and show that yes, she was in fact driving her own car. She didn’t fight the ticket.